1. Field of the Invention
The present invention relates to a sheet stacking apparatus that stacks sheets discharged from another apparatus such as an image forming apparatus.
2. Description of the Related Art
Sheet stacking apparatuses that stack a plurality of sheet-like members are used in various fields. In the field of image forming apparatuses, for example, sheet stacking apparatuses called discharge processing apparatuses are used. A discharge processing apparatus includes a plurality of stacking trays, and when one tray is fully loaded with sheets, the conveying path is switched in order to stack sheets on another tray (alternative tray). This is done because a paper jam occurs if the next sheet is discharged to the tray that has already been fully loaded with sheets. Also, in the case of electrophotographic image forming apparatuses (laser beam printers, for example), a sheet on which an image has been formed is heated to fix the image onto the sheet, so the sheet might be curled immediately after being discharged from the apparatus. Accordingly, a situation can occur in which when curl imparted to the sheet loosens after a sensor detects that the tray has been fully loaded with sheets, the detection of the fully loaded state is released, leading to an erroneous detection of the fully loaded state. In order to solve this problem, Japanese Patent Laid-Open No. 2007-153466 proposes lifting up the tray by an amount equal to a certain predetermined thickness when the fully loaded state is detected by a sensor. In other words, the amount of looseness of the curl of the sheet is canceled out by forcibly lifting up the tray, whereby it is possible to maintain the detection of the fully loaded state.